Electrical filter unit



Oct. 2, 1951 R. L. HARVEY Erm.

ELECTRICAL. FILTER UNT Filed-July 18, 194e nventor Gttomeg Rober'LlIrvey Patented Oct. 2, 1951 ELECTRICAL FILTER UNIT Robert L. Harvey and Vernon D. Landon, Princeton, N. J., assignors to Radio Corporationk of America, a corporation of Delaware Application July 1s, 1946, serial No. 684,622 claims.v (o1. 17a-44) Thisl invention relates to electrical' lter units and particularly to high-frequency filter units suited for inclusion in power supply circuits, measuring circuits, modulating circuits and the like, used with high-frequency signal generators, transmitters, receivers and other high-frequency equipment.

From the need to supply the tubes used in. highfrequency equipmentv with operating currents or voltages from a local power supply arises many problems: for example, in superheterodyne receivers, signal generators and the like, the supply leads to the oscillator tube serve undesirably to transmit, by conductionV or radiation, part of' the high-frequency oscillator output and conversely, extraneous signals picked up by the powersupply or its leads may be fed thereby into the equipment, and particularly if they are of a frequency for which some stage therein, such as an intermediate frequency amplifier stage in a superheterodyne receiver, is tuned suchsignals create troublesome interference. Generally simi-y lar problems constantly arise in other instances where conductors necessary for transmission of current of one frequency may undesirably transmit currents of other frequencies.

Among the objects of this invention is the provision of a compact filter capable of effectively attenuating one or more frequencies whose transmission by conductors provided for other purposes is to be avoided.

In accordance with the invention, all imped.- ance elements of a multisectionnlter are enclosedV Within a tubular sheath which serves as an electrical shield and which may also serve as the interposed continuation of the outer conductor of the input and output cables of the lter.

More specifically, the capacitors or shunt elements of the filter are spaced longitudinally of and Within the tubular sheath and are ofY hollow cylindrical construction or equivalent feedthrough type for passage therethrough oi'- a conductor which includes or itself forms the high impedance series elements of the filter. The plate structures of each capacitor are connected respectively to the sheath and to the inner con`` ductor between an adjacent pair of high impedance series sections: more specifically, the capacitor connections to the sheath are provided by capacitor-supporting members or discs which may also serve as spacers between aligned tubes of insulating materialv over which the sheath, when of Wire braid or like non-rigid material, is disposed.

The invention further residesv in features of construction,

combination and arrangement hereinafter described and claimed.

For more detailed understanding of the invention, and for illustration of embodiments thereof, reference is made to the accompanying drawings, in which:

Figure l is a cross-sectional View illustrating the construction of a multi-section filter unit;

Figure 2 is a detailed View, on enlarged scale, of one of the capacitor elements of Figure 1;

Figure 3 is an exploded View illustrating components of the filter of Figure 1;

Figure 4 is a fractional sectional view, on enlarged scale, showing a lter construction utilizing a inodied form` of series'impedance element;

Figure 5 isv a detailed sectional View, on enlarged scale, including a modifiedV form of capacitor element; and

Figure 6 is a sectional view of a composite multi-section filter.

Referring to Figure l, the filter unit I0 comprises a tubular outer conductor or sheath Il through whichv axially extends an inner conductor I2 which passes through a row or series of capacitors |3A-I3Z, individually supported within the outer conductor l lby a corresponding number of conductive discs [4A-MZ. The outer periphery of each of discs IAA-MZ engages and is in electrical contact with the inner surface of the tubular sheath l l and when, as in the preferred construction, the tubular sheath Il is of wire braid construction, the discs IRA-[4Z are secured thereto as by rings of solder l5, the solderflowing through the interstices of the braid and4 firmly attaching the discs to it. The discs also snugly receive the tubular capacitors and serve electrically to connect them to the sheath.

As more clearly appears in Figure 2, each of the capacitors, generically represented by capacitor |3- of Figure 2, may comprise a tube of suitable delectric I6, for example ceramic material impregnatedy with titanium, oxide, whose outer surface is covered with a metallic lm forming the outer plate Il, and whose inner surface is coated with a metallic film serving as the other condenser plate I8. The opening in each of the spacing discs USA-MZ may be such that the disc closely lits, is soldered to and makes electrical contact with the external plate structure Vl, thus tc connect one plate of each of the capacitors to the sheath il. The other plate structure of each of the capacitors is electrically connected tothe inner conductor I24 as by a drop of solder I9.

Preferably, and as already stated, the outer conductor or sheath II of the filter unit is wire braid which of itself is not sufficiently rigid to resist deformation and, therefore, the required support to prevent its collapse is provided by a series of aligned tubes A-MY, af Bakelite or other suitable insulating material, each having an outer diameter selected to suit the sheath to be used and each of suitable thickness to aiord the required strength.

The unit may be assembled by pulling the braid over a section of tubing and then alternately inM serting the discs IIIA-MZ and the other elements of tubing. The discs IA-HZ serve as spacer elements between the adjacent pairs of tubular sections and when soldered to the sheathing I I, serve to hold the tubes in place. One or more additional sheaths of wire braid, generically represented by sheath IIA, Figure 4, may be pulled over the assembly, thus in effect to attain a continuous shielding surface notwithstanding the openings in the braid of the individual sheaths.

Preferably and as shown, the terminal tubular sections 2i are tapered progressively to change the diameter of the outer conductor II from the diameter of the discs I4 to a diameter approximating that of the outer conductors or sheaths 24 and 25 of the cables to and from the lter unit. By way of example, the diameter of discs I4 for use in a filter suited to prevent passage of frequencies of the order of 1500 megacycles may be approximately one-half inch, and the capac itance of the capacitors I3 is about 50 micromicrofarads. From this information and from the proportions indicated in Figure 1, the other dimensions are readily ascertainable.

When the filter is to be used to attenuate very high frequencies, the intervals at which the capacitors are spaced within the sheathing I I is about one quarter wavelengths of the frequency to be attenuated. Attenuation at two or more frequencies is attainable by correspondingly selecting the spacing between the feed-through capacitors so that for each frequency to be attenuated there is at least one filter section which is about one quarter wavelength long at that frequency (see sections F1, F2; Fig. 6).

With wavelengths of the order of say below twenty-five centimeters the length of even a vesection filter is reasonable. When, however, a frequency to be attentuated is materially lower, the one quarter wavelength spacing would result in excessively great length of a multi-section filter. In such cases, to obtain the desired high series impedances, the conductor I2 may be coiled to form a lumped inductance between each adjacent pair of shunt capacitors. As shown in Figure 4, a coil 21 of the required number of turns, is wound upon a form 26 and the opposite terminals of the coil are connected to adjacent sections of the conductor I2 for inclusion therein.

By providing one or more sections using lumped inductance and one or more sections using the distributed inductance of a concentric line, Fig. 4, the compact self-shielded filter is suited to attenuate both very high frequencies, of the order of 1500 megacycles for example, and relatively low frequencies, of the order of 200 megacycles (see sections F3 and F1 or F2; Fig. 6). A composite lter of such nature and construction has been used in very high frequency superheterodyne receivers to attenuate frequencies corresponding with the frequencies of the oscillator and the intermediate frequency stages, so to prevent the high-frequency energy of the oscillator from being fed into the power supply conductors and to prevent any extraneous signals at intermediate frequencies picked up by the power supply from being fed into the intermediate amplifier stages.

In each of these modifications the input conductor 22 and output conductor 23 of the filter are connected to the opposite ends of the inner conductor I2 which thus serves as an interposed continuation of them. The outer conductor or sheathing of the filter may be connected to any suitable point of ground or guard potential, and in addition or alternatively, the opposite ends of the tubular conductor I I may be joined tc and form a continuation of the outer tubular conductors 24 and 25 of the input and output cables which may be used for transmission of current or power supply to the oscillator of a signal generator, transmitter, or other high-frequency equipment such as disclosed, for example, in copending applications, Serial Nos. 681,456, now abandoned, and 683,055, or it may be used for connection of a measuring circuit to a measuring device traversed by high-frequency current which it is desired to exclude from the measuring circuit. An example of such measuring circuit is disclosed in aforesaid application, Serial No. 683,055.

In Figure 5 there is disclosed another type of feed-through capacitor which is suited for use in the compact multi-section filter units of the type disclosed in Figures l and 4. In this arrangement the spacing ring I4 itself forms one of the capacitor plate structures and the other plate I8A is a fiat plate or disc spaced from the plate I4 by a thin sheet or layer of dielectric ISA, such for example as a sheet of mica. The conductor I2 passes through aligned openings in the discs and is attached as by solder I9 to the plate structure IBA. A similar arrangement may be used on the other side of disc I4 if greater capacity is required.

It is to be understood the iilter unit is not limited to use for filters whose series-impedance elements are purely inductive and whose shunt elements are purely capacitative; for example, the discs IIIA- MZ may be of resistance material so that in effect the shunt path at the junction of the series-impedance sections comprises a resistance and a capacitor in series, and/or the series-impedance coil 21 of Figure 4 may be of resistance wire. In short, the filter construction is flexible and lends itself for use in fabrication of several different types of lter networks in addition to the lowpass, shunt capacitor and series-inductance type specifically described and illustrated. Accordingly, the invention may be applied in many ways, all within the scope of the appended claims.

We claim as our invention:

1. An ultra-high frequency multi-section filter unit for physical and electrical interposition in shielded leads thereto comprising a flexible outer conductor for connection at its opposite ends to the respective outer conductors of said leads, a plurality of apertured conductive disks within and peripherally engaging said flexible outer conductor, a series of tubes of insulating material, each about one-quarter wavelength long for spacing adjacent pairs of said disks and for supporting said flexible outer conductor, a series of tubular capacitors each extending through and Supported by one of said conductive disks with its outer plate connected thereby to said outer flexible conductor, and an inner conductor for connection at its opposite ends to the inner conductors of said shielded leads and extending through said tubular capacitors with connection to Ithe inner plate of each of them.

2. A multi-section lter as in claim 1 in which the inner conductor between at least two of said disks is coiled to form a lumped inductance.

3. A multi-section iilter as in claim 1 in which the inner conductor between at least two of said disks is a straight wire about one-quarter wavelength long at a frequency to be attenuated.

4. A multi-section filter as in claim 1 in which for attenuation of different frequencies the effective electrical lengths of the inner conductor between diiierent pairs of said disks are respectively about one-quarter wavelength for said diiTerent frequencies.

5. A multi-section filter as in claim 4 in which the inner conductor between at least two of said disks is coiled to form a lumped inductance.

6. A high-frequency multi-section lter unit for physical and electrical interposition in shielded leads thereto comprising a flexible outer conductor for connection at its opposite ends to the respective outer conductors of said leads, a plurality of apertured conductive disks within and peripherally engaging said flexible outer conductor, a series of tubes of insulating material for spacing adjacent pairs of said disks and for supporting said exible outer conductor, an inner conductor for connection at its opposite ends to the inner conductors of said shielded leads and extending through said apertured disks, and a series of capacitors through which said inner conductor also extends, each of said capacitors having an electrode connected to said inner conductor and an electrode connected to said outer conductor by an associated one of said disks.

7. A multi-section lter as in claim 6 in which the inner conductor between at least two of said disks is coiled to form a lumped induotance.

8. A multi-section lter as in claim 6 in which the inner conductor between at least ltwo of said disks is a straight wire about one-quarter wavelength long at a frequency to be attenuated.

9. A multi-section lter as in claim 6 in which for attenuation of diierent frequencies the effective electrical lengths of the inner conductor between different pairs of said disks are respectively about one-quarter wavelength for said dfferent frequencies.

10. A multi-section filter as in claim 9 in which the inner conductor between at least two of said disks is coiled to form a lumped inductance.

ROBERT L. HARVEY. VERNON D. LANDON.

REFERENCES CITED The following references are of record in the le of this patent:

UNITED STATES PATENTS Number Name Date 2,030,179 Potter Feb. 11, 1936 2,109,843 Kassner Mar. 1, 1938 2,163,775 Conklin June 27, 1939 2,221,105 Otto NOV. 12, 1940 2 228,797 Wassermann Jan. 14, 1941 2,399,674 Harrison May 7, 1946 2,409,556 George Oct. 15, 1946 2,438,913 Hanse Apr. 6, 1948 2,456,803 Wheeler Dec. 21, 1948 2,479,687 Linder Aug. 23, 1949 2,491,681 Minter Dec. 20, 1949 

